Subsurface aeration seeks to release bubbles at the bottom of the pond and allow them to rise by the force of gravity. Diffused aeration systems utilize bubbles to aerate as well as mix the pond. Water displacement from the expulsion of bubbles can cause a mixing action to occur, and the contact between the water and the bubble will result in an oxygen transfer.
Bioreactors are also designed to treat sewage and wastewater. In the most efficient of these systems there is a supply of free-flowing, chemically inert media that acts as a receptacle for the bacteria that breaks down the raw sewage. Aerators supply oxygen to the sewage and media further accelerating breakdown. In the process, the liquids Biochemical Oxygen Demand BOD is reduced sufficiently to render the contaminated water fit for reuse. The biosolids are collected for further processing or dried and used as fertilizer, agricultural feed, etc.
Subsurface aeration, bioreactors and most likely a combination of both are commonly employed to treat sewage water, recycle wastewater and other water treatment applications both industrially or domestically.